The U.S. Supreme Court hears arguments Tuesday in an important case that tests how far employers must go to accommodate the religious views of their employees.

Not only does federal law make it illegal to discriminate in employment based on religion, but it also requires that employers reasonably accommodate the religious beliefs of workers as long as the accommodation would not impose an "undue hardship on the employer's business." But what is an undue hardship? Congress didn't elaborate, so the Supreme Court had to define the term.

The background to the case

Forty-six years ago, the court, by a lopsided margin, ruled that an employer need not accommodate a worker's desire to avoid work on the Sabbath if that would mean operating short-handed or regularly paying premium wages to replacement workers. The court went on to say that employers should not have to bear more than what it called a "de minimis," or trifling, cost. That "de minimis" language has sparked a lot of criticism over the years. But Congress has repeatedly rejected proposals to provide greater accommodations for religious observers, including those who object to working on the Sabbath.

Now, however, religious groups of every kind are pressing a new group of more conservative justices to overturn or modify the court's earlier ruling.

At the center of the case is Gerald Groff, an evangelical Christian.

"I believe in a literal keeping of the Lord's Day," Groff said. "It's the entire day as a day of rest and ... spending time with fellow believers. But most of all, just to honor God and keep the day special unto him," he says.

Starting in 2012, Groff worked for the U.S. Postal Service as a carrier associate in rural Pennsylvania. These rural carriers are non-career employees who fill in for more senior career employees during absences. Initially, Groff had no problem, because rural carriers were not required to work on Sundays. But in 2013, the Postal Service signed a contract with Amazon to deliver its packages, and that, of course, meant Sunday deliveries.

In a contract negotiated with the union, the Postal Service established a process for scheduling employees for Sunday and holiday Amazon deliveries. The process first called for non-career employees like Groff to fill in the gaps. Then, volunteers willing to work Sundays and holidays would be called, and if none of this was sufficient to meet demand, the rural associate and assistant carriers would be assigned on a regular rotating basis.

The problem for Groff was that he didn't want to ever work Sundays, and the problem for the Postal Service was — and is — that it is chronically understaffed, especially in rural areas. To solve that problem, the Postal Service pools its employees from multiple post offices in a rural area to work on a regular Sunday rotation.

Groff, facing potential disciplinary action for refusal to report for Sunday work, quit and sued the Postal Service for failure to accommodate his religious views. Representing him is the First Liberty Institute, a conservative Christian organization. It is asking the court to throw out its 1977 decision and declare that an undue hardship would have to be a "significant difficulty or expense," instead of "more than a de minimis cost to a business."

"They would have to pay him overtime anyway," Hiram Sasser, First Liberty's general counsel said. "So there's no extra expense."

USPS' argument

The Postal Service counters that Groff's lawyers are mischaracterizing the way the court's 1977 decision has been applied in practice. Just three years after the decision, the Equal Employment Opportunity Commission issued rules further defining what an undue hardship means — rules that are more deferential to the religious views of employees.

The Postal Service contends that under those more generous rules, accommodating Groff still would have imposed an undue hardship on the Postal Service as a business by requiring it to operate with insufficient staff in a manner that would so burden other employees that substantial numbers would transfer or quit their jobs. The Postal Service argues that this qualifies as an undue hardship on its business under any standard.

Tuesday's argument will, of course, be before a court that is dramatically different from the court that decided what it means to accommodate religious views in the workplace nearly a half-century ago. That court sought to balance burdens, while the current court has consistently and explicitly shifted the balance to favor religiously observant groups, whether those groups are religious employers or religious employees.